Imaging device

ABSTRACT

An imaging device includes a first image pickup cell having a first photoelectric converter converting incident light into first charges, the first photoelectric converter including a first pixel electrode, a first electrode, and a first photoelectric conversion region between the first pixel electrode and the first electrode, and a first charge storage node coupled to the first pixel electrode for accumulating the first charges; a second image pickup cell having a second photoelectric converter converting incident light into second charges, the second photoelectric converter including a second pixel electrode, a second electrode, and a second photoelectric conversion region between the second pixel electrode and the second electrode, and a second charge storage node coupled to the second pixel electrode for accumulating the second charges. The first pixel electrode has a first area, and the second pixel electrode has a second area less than the first area.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/946,448, filed on Apr. 5, 2018, now allowed, which is a Continuationof U.S. patent application Ser. No. 14/857,699, filed on Sep. 17, 2015,now U.S. Pat. No. 9,967,501, which in turn claims the benefit ofJapanese Application No. 2015-165895, filed on Aug. 25, 2015 andJapanese Application No. 2014-207305, filed on Oct. 8, 2014, the entiredisclosures of which Applications are incorporated by reference herein.

BACKGROUND Technical Field

The present disclosure relates to an imaging device including aphotoelectric conversion film.

Description of the Related Art

Subjects present in the natural world have a high dynamic range. Forexample, in the case of an in-car imaging device, the brightness of asubject changes from moment to moment, and the in-car imaging device isrequired to simultaneously pick up images of a bright subject and a darksubject (have a high dynamic range). To achieve a high dynamic range,Japanese Unexamined Patent Application Publications Nos. 62-108678 and2008-99073 propose the methods below.

The imaging devices disclosed in Japanese Unexamined Patent ApplicationPublications Nos. 62-108678 and 2008-99073 use a silicon photodiode. InJapanese Unexamined Patent Application Publication No. 62-108678, imagesdifferent in exposure time (which may also be referred to as “storagetime” hereinafter) are synthesized, which allows achievement of a highdynamic range. The approach has already been put to practical use. InJapanese Unexamined Patent Application Publication No. 2008-99073, adynamic range is extended by synthesizing images obtained from aplurality of image pickup cells different in sensitivity which arearranged within one pixel.

Japanese Unexamined Patent Application Publication No. 2007-59465proposes a laminated sensor including a photoelectric conversion filminstead of a silicon photodiode which may become an obstacle to a highdynamic range.

SUMMARY

The above-described conventional imaging devices are expected to achievefurther improvement in high dynamic range photographing.

In one general aspect, the techniques disclosed here feature an imagingdevice comprising: a first image pickup cell comprising a firstphotoelectric converter that converts incident light into first charges,the first photoelectric converter including a first pixel electrode, afirst electrode facing the first pixel electrode, and a firstphotoelectric conversion region sandwiched between the first pixelelectrode and the first electrode, and a first charge storage node thatis coupled to the first pixel electrode, the first charge storage nodeaccumulating the first charges; a second image pickup cell adjacent tothe first pixel, the second image pickup cell comprising a secondphotoelectric converter that converts incident light into secondcharges, the second photoelectric converter including a second pixelelectrode, a second electrode facing the second pixel electrode, and asecond photoelectric conversion region sandwiched between the secondpixel electrode and the second electrode, and a second charge storagenode that is coupled to the second pixel electrode, the second chargestorage node accumulating the second charges; and a first microlenslocated on an incident light side of the first photoelectric converter,the first pixel electrode being located on an optical axis of the firstmicrolens, wherein the first pixel electrode has a first area, and thesecond pixel electrode has a second area less than the first area.

It should be noted that general or specific embodiments may beimplemented as an element, a device, a system, an integrated circuit, amethod, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments willbecome apparent from the specification and drawings. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart for explaining the difference between conventionalimage pickup cell characteristics and desirable image pickup cellcharacteristics;

FIG. 2 is a chart for explaining the difference between the conventionalimage pickup cell characteristics and more desirable image pickup cellcharacteristics;

FIG. 3 is a schematic chart showing the relations between thecapacitance of a charge storage node and the number of saturationelectrons (e⁻) and random noise (e⁻);

FIG. 4 is a schematic diagram showing an example of the structure of animaging device according to a first exemplary embodiment;

FIG. 5 is a schematic diagram showing the circuit configuration of aunit pixel;

FIG. 6 is a schematic cross-sectional view showing the deviceconfiguration of the unit pixel;

FIG. 7 is a schematic view showing the planar shapes of pixel electrodeswithin the unit pixel when viewed from a normal direction of a P-typesilicon substrate;

FIG. 8 is a graph showing a light condensing ratio characteristic of asecond pixel electrode when the radius of the second pixel electrode isvaried if the light condensing ratio of the whole unit pixel isstandardized at 100%;

FIG. 9 is a schematic view showing areas occupied by a first chargedetection circuit and a second charge detection circuit in the unitpixel;

FIG. 10 is an operation sequence chart showing exposure and readoutoperations during one cycle period in the imaging device;

FIG. 11A is a schematic view showing a planar shape of the second pixelelectrode having a doughnut shape as a pixel electrode variant;

FIG. 11B is a schematic view showing a planar shape of the second pixelelectrode having a cross shape as a pixel electrode variant;

FIG. 11C is a schematic view showing a planar shape of the second pixelelectrode having a notched shape as a pixel electrode variant;

FIG. 12A is a schematic view showing respective examples of the shapesof the pixel electrodes without a microlens;

FIG. 12B is a schematic view showing other respective examples of theshapes of the pixel electrodes without the microlens;

FIG. 13 is a schematic cross-sectional view showing the deviceconfiguration of a unit pixel according to a second exemplaryembodiment;

FIG. 14 is a schematic view with a focus on 3-by-3 unit pixels accordingto a modification of the second embodiment that shows how the unitpixels are laid out in an imaging device;

FIG. 15 is a schematic cross-sectional view showing the deviceconfiguration of the unit pixel according to the modification of thesecond exemplary embodiment;

FIG. 16 is a schematic cross-sectional view showing a cross-section ofthe unit pixel taken along line XVI-XVI shown in FIG. 14;

FIG. 17 is a schematic cross-sectional view showing the deviceconfiguration of a unit pixel according to another modification of thesecond exemplary embodiment; and

FIG. 18 is a diagram that schematically shows functional blocks of animage pickup module equipped with an imaging device according to a thirdexemplary embodiment.

DETAILED DESCRIPTION

Problems of the related art considered by the inventor of the presentdisclosure will be described first.

In image merging disclosed in Japanese Unexamined Patent ApplicationPublication No. 62-108678, a plurality of pieces of image data aresequentially acquired. For this reason, a time several times as much asa normal image pickup time is required to obtain one composite image.Since images at different times are synthesized, simultaneity of imagesis lost, and a distortion may appear in an image of a moving subject.

In Japanese Unexamined Patent Application Publication No. 2008-99073,the imaging device includes a plurality of photodiodes that have thesame size, the same sensitivity, and the same number of saturationelectrons. The imaging device further includes on-chip top lenses thatdivide incident light into two kinds of light that are strong light andweak light. And the on-chip top lenses cause any one of the two kinds oflight to enter each of the photodiodes. With this configuration, theimage pickup cells are effectively considered to have two differentsensitivities. Since these two cells are mounted on one pixel,simultaneous image pickup is possible, and simultaneity of images issecured.

Since two cells need to be arranged within one pixel, the area of eachphotodiode is inevitably one-half or less of that of a conventionalphotodiode. The area of each photodiode is approximately proportional tothe sensitivity or the number of saturation electrons. If the area ofeach photodiode is one-half or less, the sensitivity and the number ofsaturation electrons are each one-half or less of that of a conventionalphotodiode.

FIG. 1 schematically shows conventional image pickup cellcharacteristics and desirable image pickup cell characteristics. Theabscissa represents sensitivity while the ordinate represents the numberof saturation electrons. Sensitivity here is one of indicators ofcharacteristics of an imaging device (image sensor) and refers to thenumber of charges (electron-hole pairs) generated in an image pickupcell upon receipt of incident light. Sensitivity is generally expressedin the unit (e⁻/Lux·sec). The number of saturation electrons refers to apermissible amount for electrons stored in an image pickup cell and isexpressed in the unit (e⁻). Sensitivity and the number of saturationelectrons are proportional to the effective area of a photoelectricconversion element in principle. Note that sensitivity also depends on amicrolens design.

Unlike a commonly-used cell (hereinafter referred to as a “common cell”)which has one image pickup cell within a single pixel, high dynamicrange (HDR) photographing uses two image pickup cells within a singlepixel. The two image pickup cells desirably have (a) image pickup cellcharacteristics with a sensitivity and the number of saturationelectrons comparable to those of a common cell and (b) image pickup cellcharacteristics with the number of saturation electrons comparable tothat of a common cell and a sensitivity lower than that of a commoncell. Reference characters a and b in FIG. 1 denote a desirablecombination.

Reference characters a′ and b′ in FIG. 1 denote a combination of twoimage pickup cells in Japanese Unexamined Patent Application PublicationNo. 2008-99073. As described above, the area of each image pickup cell(photodiode) is not more than one-half of that of a common cell. Forthis reason, the sensitivity of each image pickup cell is lower, and thenumber of saturation electrons is smaller. This means divergence fromdesirable characteristics. As described above, the image pickup cellcharacteristics in Japanese Unexamined Patent Application PublicationNo. 2008-99073 are significantly inferior to required characteristics.

FIG. 2 schematically shows the conventional image pickup cellcharacteristics and more desirable image pickup cell characteristics. Areduction in sensitivity as indicated by b in FIG. 1 can preventsaturation even if the amount of incident light is large. Additionally,if the number of saturation electrons itself can be increased asindicated by b in FIG. 2, a dynamic range is further extended.

Table below compares a conventional Si sensor having a photodiode withthe laminated sensor having a photoelectric conversion film disclosed inJapanese Unexamined Patent Application Publication No. 2007-59465 andshows factors which determine element functions and sensor performanceof each sensor. As can be seen from Table, in the conventional Sisensor, sensitivity and the number of saturation electrons are bothdetermined by the performance of a photodiode. In contrast, in thelaminated sensor having the photoelectric conversion film, sensitivitydepends on the area and the quantum efficiency of the photoelectricconversion film, and the number of saturation electrons depends on thecapacitance of a charge storage node. For this reason, the number ofsaturation electrons increases with an increase in the capacitance ofthe charge storage node. The number of saturation electrons can beincreased independently of the performance of the photoelectricconversion film in the laminated sensor. However, if the capacitance ofthe charge storage node is increased, major side effects occurs.

TABLE Photoelectric Conventional conversion film Si sensor sensorElement Photoelectric Photodiode Photoelectric functions conversionconversion film Charge storage Photodiode Charge storage node Signalreadout Floating Charge storage node (gate voltage of diffusionamplification transistor) Sensor Sensitivity Dependent on Dependent onarea performance quantum and quantum efficiency efficiency of photodiodeof photoelectric conversion film Number of Dependent on Dependent onsaturation capacitance of capacitance of charge electrons photodiodestorage node

FIG. 3 schematically shows the relations between the capacitance of acharge storage node and the number of saturation electrons (e⁻) andrandom noise (e⁻). The abscissa represents the capacitance of a chargestorage node while the ordinate represents the number of saturationelectrons and random noise. The number of saturation electrons can beincreased by increasing the capacitance of a charge storage node. Theincrease in the number of saturation electrons, however, also causes theproblem of an increase in random noise.

Random noise mainly includes, for example, noise which is generated whena charge detection circuit reads out charges stored in a charge storagenode, that is, when the charge detection circuit transfers the chargesand noise which is generated when the charge detection circuit resetscharges stored in the charge storage node (hereinafter referred to as“reset noise”). Although the number of saturation electrons can beincreased by making the capacitance of the charge storage node large,the ratio of the variation in the number of stored charges to thevariation in the voltage of the charge storage node increases. Sincenoise generated in the charge detection circuit is voltage noise, thenoise converted into the number of stored charges turns out to be largeconsequently.

In a sensor using a silicon photodiode, complete transfer of charges isperformed, and correlated double sampling (CDS) is effective insuppressing reset noise. In contrast, in a laminated sensor using aphotoelectric conversion film, complete transfer of charges isimpossible, and reset noise cannot be cancelled out even with use ofCDS. Although the details will be described later, noise canceling usingfeedback as proposed in, for example, Japanese Unexamined PatentApplication Publication No. 2012-19167, is thus necessary. As describedabove, an increase in the capacitance of a charge storage node increasesthe ratio of the variation in the number of stored charges to thevariation in the voltage of the charge storage node. For this reason,reset noise cannot be sufficiently suppressed even by feedback.

Embodiments of the present disclosure will be described below withreference to the drawings. Note that the present disclosure is notlimited to the embodiments below. Appropriate changes may be madewithout departing from the scope of the present disclosure. Oneembodiment can be combined with another embodiment. In the descriptionbelow, identical or similar components are denoted by identicalreference characters. A redundant description of the components may beomitted.

First Embodiment

The structure of, functions of, and a driving method for an imagingdevice 100 according to the present embodiment will be described withreference to FIGS. 4 to 12B. An example using a P-type silicon substrateas a semiconductor substrate will be described below. An example using ahole as a signal charge will be illustrated. Note that an electron maybe used as a signal charge.

(Structure of Imaging Device 100)

The structure of the imaging device 100 will be described first withreference to FIG. 4.

FIG. 4 schematically shows an example of the structure of the imagingdevice 100. The imaging device 100 includes a plurality of unit pixels30 which are two-dimensionally arrayed. Note that although severalmillions of unit pixels 30 can be two-dimensionally arrayed in practice,FIG. 4 shows the unit pixels 30 arranged in a 2-by-2 matrix of thetwo-dimensionally arrayed unit pixels 30. Note that the imaging device100 may be a line sensor. In this case, the plurality of unit pixels 30are one-dimensionally arrayed (in a row direction or a columndirection).

The unit pixel 30 includes a first image pickup cell 31 and a secondimage pickup cell 31′. The first image pickup cell 31 is an image pickupcell for a high level of saturation. The second image pickup cell 31′ isan image pickup cell for low noise. Typically, the first image pickupcell 31 functions as a low-sensitivity image pickup cell while thesecond image pickup cell 31′ functions as a high-sensitivity imagepickup cell. The imaging device 100 includes, for each first imagepickup cell 31, a reset signal line 47 and an address signal line 48which are arranged for each row and a vertical signal line 45, a powerwiring 46, and a feedback signal line 49 which are arranged for eachcolumn. The imaging device 100 also includes, for each second imagepickup cell 31′, a reset signal line 47′ and an address signal line 48′which are arranged for each row and a vertical signal line 45′, a powerwiring 46, and a feedback signal line 49′ which are arranged for eachcolumn.

The imaging device 100 separately includes a first peripheral circuitwhich processes a signal from the first image pickup cell 31 and asecond peripheral circuit which processes a signal from the second imagepickup cell 31′. The first peripheral circuit has a first verticalscanning circuit 52, a first horizontal scanning circuit 53, and a firstcolumn AD conversion circuit 54. The second peripheral circuit has asecond vertical scanning circuit 52′, a second horizontal scanningcircuit 53′, and a second column AD conversion circuit 54′. Note that acommon address signal line can be used as an address signal line for thefirst image pickup cell 31 and an address signal line for the secondimage pickup cell 31′, depending on the configuration of each pixel.

A description will be given with a focus on the first image pickup cell31. The first vertical scanning circuit 52 controls a plurality of resetsignal lines 47 and a plurality of address signal lines 48. The verticalsignal line 45 is connected to the first horizontal scanning circuit 53to transmit a pixel signal to the first horizontal scanning circuit 53.The power wiring 46 supplies a power voltage to all the unit pixels 30.The feedback signal line 49 transmits a feedback signal from a feedbackamplifier 50 (to be described later) to the first image pickup cell 31of the unit pixel 30. As for the second image pickup cell 31′, thecircuits corresponding to the second image pickup cell 31′ control thesignal lines corresponding to the second image pickup cell 31′, like thefirst image pickup cell 31.

(Circuit Configuration of First and Second Image Pickup Cells 31 and31′)

Respective examples of the circuit configurations of the first andsecond image pickup cells 31 and 31′ will be described with reference toFIG. 5. Note that the first and second image pickup cells 31 and 31′have substantially the same circuit configurations that are independentof each other.

FIG. 5 is an enlarged diagram of the unit pixel 30 and schematicallyshows the circuit configurations of the first and second image pickupcells 31 and 31′. The first image pickup cell 31 includes a firstphotoelectric converter 43 and a first charge detection circuit 51. Thesecond image pickup cell 31′ includes a second photoelectric converter43′ and a second charge detection circuit 51′. The circuitconfigurations will be described below with a focus on the first imagepickup cell 31.

The first charge detection circuit 51 includes an amplificationtransistor 40, a reset transistor 41, and an address transistor 42.

The first photoelectric converter 43 is electrically connected to adrain electrode of the reset transistor 41 and a gate electrode of theamplification transistor 40. The first photoelectric converter 43converts light (incident light) incident on the first image pickup cell31 into a charge. The first photoelectric converter 43 generates asignal charge corresponding to the amount of incident light. Thegenerated signal charge is stored in a charge storage node 44.

The power wiring 46 is connected to a source electrode of theamplification transistor 40. The power wiring 46 is extended in thecolumn direction for the reason below. The first image pickup cells 31are selected on a row-by-row basis. If the power wiring 46 is extendedin the row direction, pixel driving currents for one row all flowthrough one power wiring 46 to cause a large voltage drop. With thepower wirings 46, a common source follower power voltage is applied tothe amplification transistors 40 within all the first image pickup cells31 in the imaging device 100.

The amplification transistor 40 amplifies a signal voltage correspondingto the amount of signal charges stored in the charge storage node 44.

A gate electrode of the reset transistor 41 is connected to the firstvertical scanning circuit 52 via the reset signal line 47. A sourceelectrode of the reset transistor 41 is connected to the feedback signalline 49. The reset transistor 41 resets (initializes) charges stored inthe charge storage node 44. In other words, the reset transistor 41resets a potential at the gate electrode of the amplification transistor40.

A gate electrode of the address transistor 42 is connected to the firstvertical scanning circuit 52 via the address signal line 48. A drainelectrode of the address transistor 42 is connected to the firsthorizontal scanning circuit 53 via the vertical signal line 45. Theaddress transistor 42 selectively outputs an output voltage from theamplification transistor 40 to the vertical signal line 45.

The first vertical scanning circuit 52 applies a row selection signalfor on-off control of the address transistor 42 to the gate electrode ofthe address transistor 42. With this application, scanning for a row tobe read out is performed in a vertical direction (the column direction),and a row to be read out is selected. Signal voltages are read out fromthe first image pickup cells 31 of the unit pixels 30 in the selectedrow to the vertical signal lines 45. The first vertical scanning circuit52 applies a reset signal for on-off control of the reset transistor 41to the gate electrode of the reset transistor 41. With this application,a row as a reset operation object with the first image pickup cells 31of the unit pixels 30 is selected.

The first column AD conversion circuit 54 performs noise suppressionsignal processing, typified by, for example, correlated double sampling,and analog-to-digital conversion (AD conversion) on signals read outfrom the first image pickup cells 31 to the vertical signal line 45 on arow-by-row basis. The first horizontal scanning circuit 53 reads out asignal processed by the first column AD conversion circuit 54.

(Device Structure of Unit Pixel 30)

FIG. 6 schematically shows a cross-section of the device structure ofthe unit pixel 30 in the imaging device 100 according to the presentembodiment.

The unit pixel 30 typically has a P-type silicon substrate 1, the firstimage pickup cell 31, the second image pickup cell 31′, a photoelectricconversion film 9, an upper electrode 10, a color filter 11, and amicrolens 12. Note that the color filter 11 need not be provided if onlymonochrome image pickup is performed. If light condensing by a microlensis not performed, the microlens 12 need not be provided.

A photoelectric converter is composed of the photoelectric conversionfilm 9, the upper electrode 10, a first pixel electrode 7, and a secondpixel electrode 8. The photoelectric converter has the firstphotoelectric converter 43 of the first image pickup cell 31 and thesecond photoelectric converter 43′ of the second image pickup cell 31′.The photoelectric conversion film 9 includes a first photoelectricconversion region 33 for the first image pickup cell 31 and a secondphotoelectric conversion region 33′ for the second image pickup cell31′. The first photoelectric conversion region 33 is in contact with thefirst pixel electrode 7, and the second photoelectric conversion region33′ is in contact with the second pixel electrode 8. In the presentembodiment, the sensitivity of the first image pickup cell 31 is lowerthan that of the second image pickup cell 31′. The capacitance of thecharge storage node of the first image pickup cell 31 is larger thanthat of a charge storage node of the second image pickup cell 31′.

It seems in the cross-sectional view of FIG. 6 that the first pixelelectrode 7 is divided into two portions and that the second pixelelectrode 8 is sandwiched between the two portions. However, the twoportions of the first pixel electrode 7 in FIG. 6 have the sameelectrical potential and constitute a single pixel electrode.

The microlens 12 is supported by the P-type silicon substrate 1 so as tocover the whole photoelectric converter. As described above, the firstimage pickup cell 31 and the second image pickup cell 31′ have thecommon microlens 12. The microlens 12 condenses light incident on theunit pixel 30 onto a center (the second pixel electrode 8) of the unitpixel 30. The second pixel electrode 8 may be arranged on an opticalaxis of the microlens 12.

The photoelectric conversion film 9 is stacked above the P-type siliconsubstrate 1. The photoelectric conversion film 9 can be formed of, forexample, an organic material or amorphous silicon. The photoelectricconversion film 9 converts incident light from the outside into acharge. The first pixel electrode 7 and the second pixel electrode 8 arein contact with a surface on the P-type silicon substrate 1 side of thephotoelectric conversion film 9. In other words, the first pixelelectrode 7 and the second pixel electrode 8 are arranged between theP-type silicon substrate 1 and the photoelectric conversion film 9. Thefirst pixel electrode 7 collects signal charges generated in the firstphotoelectric conversion region 33. The second pixel electrode 8collects signal charges generated in the second photoelectric conversionregion 33′.

The upper electrode 10 is a transparent electrode and is formed incontact with an opposite surface of the second pixel electrode 8 fromthe first pixel electrode 7 and the second pixel electrode 8. A firstpositive fixed voltage is applied to the upper electrode 10. A secondpositive fixed voltage smaller than the first positive fixed voltage isapplied to the first pixel electrode 7 and the second pixel electrode 8.An electron-hole pair is generated in the photoelectric conversion film9 through photoelectric conversion. A hole generated in the firstphotoelectric conversion region 33 on the first pixel electrode 7 movesto the first pixel electrode 7. A hole generated in the secondphotoelectric conversion region 33′ on the second pixel electrode 8moves to the second pixel electrode 8.

The first image pickup cell 31 has a region for the unit pixel 30 exceptfor a region for the second image pickup cell 31′. The first imagepickup cell 31 includes the first photoelectric conversion region 33,the first pixel electrode 7, a first interconnect 32, the first chargedetection circuit 51, and a shallow trench isolation (STI) layer 2.

The first charge detection circuit 51 is formed on the P-type siliconsubstrate 1. The first charge detection circuit 51 is electricallyconnected to the first pixel electrode 7 via the first interconnect 32.A first diffusion layer 22 in FIG. 6 is an N-type source region of thefirst reset transistor 41 (see FIG. 5). An arrow indicates a gate widthof the first amplification transistor 40 (see FIG. 5). Note that drainand source regions and the like of the first amplification transistor 40are arranged in a direction perpendicular to the sheet surface and arenot shown.

The first interconnect 32 stores charges (holes) which have moved to thefirst pixel electrode 7. Besides the first interconnect 32, the firstpixel electrode 7, the first diffusion layer 22, a gate electrode 3 ofthe first amplification transistor 40, local interconnect 4, contactplug 5, and other interconnects (not shown) which electrically connectthe components can also function as charge storage nodes which store theholes. These things which functions as charge storage nodes arecollectively called the “charge storage node 44” (see FIG. 5). The gateelectrode 3 can be formed of polysilicon.

The first image pickup cell 31 further includes a metal oxide metal(MOM) capacitor 6 one end of which is electrically connected to thefirst pixel electrode 7. A fixed voltage, for example, a ground voltageis applied to the other end of MOM capacitor 6. The MOM capacitor 6increases the capacitance of the charge storage node 44. This results inan increase in the number of saturation electrons of the first imagepickup cell 31, as shown in FIG. 3. The first image pickup cell 31functions as an image pickup cell for a high level of saturation.

The second image pickup cell 31′ includes the second photoelectricconversion region 33′, the second pixel electrode 8, a secondinterconnect 32′, the second charge detection circuit 51′, and the STIlayer 2.

The second charge detection circuit 51′ is formed on the P-type siliconsubstrate 1. The second charge detection circuit 51′ is electricallyconnected to the second pixel electrode 8 via the second interconnect32′. A second diffusion layer 23 in FIG. 6 is an N-type source region ofa second reset transistor 41′ (see FIG. 5).

The second interconnect 32′ stores holes which have moved to the secondpixel electrode 8. Besides the second interconnect 32′, the second pixelelectrode 8, the second diffusion layer 23, the gate electrode 3 of asecond amplification transistor 40′, local interconnect 4, contact plug5, and other interconnects (not shown) which electrically connect thecomponents can also function as charge storage nodes which store holes.These things which functions as charge storage nodes are collectivelycalled a “charge storage node 44′” (see FIG. 5).

The second image pickup cell 31′ is not provided with a capacitiveelement, such as a MOM capacitor. As shown in FIG. 3, random noise canbe suppressed by making the capacitance of the charge storage node 44′of the second image pickup cell 31′ relatively smaller. The second imagepickup cell 31′ functions as an image pickup cell for low noise.

The first interconnect 32 and the second interconnect 32′ are connectedto local interconnect 4 via contact plugs 5. The first interconnect 32is electrically connected to the gate electrode 3 and the firstdiffusion layer 22 via the local interconnect 4. The second interconnect32′ is electrically connected to the gate electrode 3 and the seconddiffusion layer 23 via the local interconnect 4. Note that the localinterconnect 4 can be formed of polysilicon.

FIG. 7 shows the planar shapes of the pixel electrodes (the first pixelelectrode 7 and the second pixel electrode 8) within the unit pixel 30when viewed from a normal direction of a P-type silicon substrate 1. Thesecond pixel electrode 8 is arranged at the center of the unit pixel 30and has a substantially circular shape. The radius of the second pixelelectrode 8 is, for example, 0.75 μm. The first pixel electrode 7 isarranged so as to surround the second pixel electrode 8 with a gapbetween the first pixel electrode 7 and the second pixel electrode 8.The area of the second pixel electrode 8 is smaller than that of thefirst pixel electrode 7.

A length W of one side of the unit pixel 30 is, for example, 3 μm. Theunit pixel 30 includes Cu interconnects having three layers. The lengthW corresponds to a distance (pixel pitch) between the centers of theadjacent unit pixels 30.

In the present embodiment, the area of the first pixel electrode 7 islarger than that of the second pixel electrode 8. The second pixelelectrode 8 is arranged in a region where light is condensed by themicrolens 12 (the vicinity of the center of the unit pixel 30). Withthis arrangement, the second image pickup cell 31′ with the smaller areais made to function as a high-sensitivity image pickup cell, and thefirst image pickup cell 31 is made to function as a low-sensitivityimage pickup cell, through utilization of light condensing by themicrolens 12. As a result, the first image pickup cell 31 can pick up alow-sensitivity image, and the second image pickup cell 31′ can pick upa high-sensitivity image. A high-sensitivity image refers to, forexample, an image of a dark subject which is obtained in a darkenvironment. A low-sensitivity image refers to, for example, an image ofa bright subject which is obtained in a bright environment.

The sensitivities of the first image pickup cell 31 and the second imagepickup cell 31′ will be described in more detail with reference to FIG.8.

FIG. 8 shows the relation between the radius of the second pixelelectrode 8 and the light condensing ratio of the second pixel electrode8 when the light condensing ratio of the whole unit pixel 30 isstandardized at 100%. The abscissa represents the radius (μm) of thesecond pixel electrode 8 while the ordinate represents the lightcondensing ratio (%) of the second pixel electrode 8.

If the radius of the second pixel electrode 8 is 0.75 μm, the area ofthe second pixel electrode 8 is about 20% of the area of the whole unitpixel 30. It can be seen that a high light condensing ratio of 90% ormore is achieved even in this case. This is because incident light iscondensed mainly onto the center of the pixel by the microlens 12. Thelight condensing ratio is proportional to the number of charges (holes)generated in the photoelectric conversion film located on the pixelelectrode. As long as light is condensed by the microlens 12, a highsensitivity of 90% or more is maintained even if the area of the secondpixel electrode 8 is small.

In contrast, the area of the first pixel electrode 7 occupies 80% of thearea of the whole unit pixel 30. However, only light that is not morethan 10% of incident light is incident on the first photoelectricconversion region 33 on the first pixel electrode 7. For this reason,the sensitivity of the first image pickup cell 31 decreases to 10% orless. As described above, a sensitivity difference of about one order ofmagnitude can be produced between the first image pickup cell 31 and thesecond image pickup cell 31′.

Note that materials generally used to manufacture silicon semiconductordevices can be extensively used as the materials for the electrodes ofthe unit pixel 30 and the interconnects.

Referring back to FIG. 5, the description will be continued.

In the imaging device 100, random noise may be generated when a signalcharge is transferred or reset. Note that random noise resulting mainlyfrom reset noise which is generated when a signal charge is reset willbe described below.

If random noise remains after resetting, the remaining noise may beadded to a signal charge subsequently stored in the charge storage node44. In this case, when the signal charge is read out, a signal withrandom noise superimposed thereon is output.

The imaging device 100 includes a feedback circuit to reduce theabove-described random noise. The feedback operation of the feedbackcircuit will be described below.

The feedback circuit includes the feedback amplifier 50. The feedbackamplifier 50 is provided so as to correspond to each column of firstimage pickup cells 31 of the unit pixels 30. A negative input terminalof the feedback amplifier 50 is connected to the corresponding verticalsignal line 45. An output terminal of the feedback amplifier 50 and thesource electrode of the reset transistor 41 are connected by thefeedback signal line 49 via a switch. Thus, when the amplificationtransistor 40, the address transistor 42, and the reset transistor 41are conducting, an output value from the address transistor 42 is inputto the negative terminal of the feedback amplifier 50. The feedbackamplifier 50 performs feedback operation such that the gate potential ofthe amplification transistor 40 is a predetermined feedback voltage.

In the imaging device 100, the first image pickup cells 31 within theunit pixels 30 in one row are selected by the first vertical scanningcircuit 52. A charge signal obtained through photoelectric conversion bythe first photoelectric converter 43 in each selected unit pixel 30 isamplified by the amplification transistor 40. The amplified signal isoutput to the vertical signal line 45 via the address transistor 42.

The output signal is output to the outside via the first horizontalscanning circuit 53. A signal charge in the first image pickup cell 31is discharged when the reset transistor 41 is turned on. A large thermalnoise (random noise) called kTC noise is generated from the resettransistor 41 at the time of the discharge. The thermal noise remains inthe charge storage node 44 even after reset operation.

In order to suppress thermal noise, the vertical signal line 45 isconnected to the negative input terminal of the feedback amplifier 50. Avoltage value to the negative input terminal is reversely amplified bythe feedback amplifier 50. When a charge in the charge storage node 44is reset by the reset transistor 41, the three transistors are broughtinto conduction. The reversely amplified signal is fed back to thesource electrode of the reset transistor 41 via the feedback signal line49. More specifically, random noise generated in the charge storage node44 is negatively fed back to a source electrode of the reset transistor41 via the amplification transistor 40, the address transistor 42, thevertical signal line 45, the feedback amplifier 50, and the feedbacksignal line 49. In the above-described manner, a noise component in thecharge storage node 44 is canceled out. That is, negative feedbackcontrol allows suppression of random noise. Note that an AC component ofthermal noise is fed back to the source electrode of the resettransistor 41. A DC component is a positive voltage close to 0 V.

As described above, the number of saturation electrons is determined bythe capacitance of the charge storage node 44 that stores charges(holes) generated in the photoelectric conversion film 9.

Referring back to FIG. 6, the description will be continued.

The capacitance of the charge storage node 44 in the first image pickupcell 31 mainly includes a capacitance between the first pixel electrode7 and the upper electrode 10, a capacitance between the first pixelelectrode 7 and the second pixel electrode 8, a capacitance between thefirst pixel electrode 7 and the first pixel electrode 7 of the adjacentunit pixel 30, a parasitic capacitance of Cu interconnect 32, a gatecapacitance of the first amplification transistor 40, and a junctioncapacitance of the first diffusion layer 22. The capacitance of thecharge storage node 44′ in the second image pickup cell 31′ mainlyincludes a capacitance between the second pixel electrode 8 and theupper electrode 10, a capacitance between the first pixel electrode 7and the second pixel electrode 8, a parasitic capacitance of Cuinterconnect 32′, a gate capacitance of the second amplificationtransistor 40′, and a junction capacitance of the second diffusion layer23. Among the capacitances, capacitances which make up large proportionsof the capacitances of the charge storage nodes 44 and 44′ are onesrelated to the first pixel electrode 7 and the second pixel electrode 8.

The first image pickup cell 31 functions as an image pickup region forpickup of an image of a bright subject with a large amount of light. Adesirable characteristic required of the first image pickup cell 31 isthe largeness of the number of saturation electrons (a high level ofsaturation). In the present embodiment, the first pixel electrode 7 isarranged so as to avoid a central region of the unit pixel 30 whereincident light is condensed by the microlens 12, as shown in FIG. 7.That is, the first pixel electrode 7 is arranged in a peripheral regionof the unit pixel 30. The area of the first pixel electrode 7 can thusbe sufficiently large. As a result, the capacitance of the chargestorage node 44 can be increased, and the desirable characteristic for ahigh level of saturation can be achieved.

The capacitance of the charge storage node 44 of the first image pickupcell 31 is further made larger by electrically connecting the MOMcapacitor 6 to the first pixel electrode 7, as shown in FIG. 6. In theunit pixels 30 adjacent to each other, since the charge storage nodes44′ of the respective second image pickup cells 31′ are separated fromeach other, the degree of electrical capacitive coupling is low.However, the second image pickup cell 31′ is an image pickup cell forlow noise, and color mixing due to a weak capacitive coupling isunacceptable. Arrangement of the MOM capacitor 6 between Cuinterconnects 32′ of the two second image pickup cells 31′ allowssuppression of color mixing due to a capacitive coupling between thecharge storage nodes 44′ and allows low-noise image pickup.

Note that the MOM capacitor 6 increases a capacitive coupling betweenthe charge storage node 44 of the first image pickup cell 31 and thecharge storage node 44′ of the second image pickup cell 31′ in thesingle unit pixel 30.

However, the charge storage node 44 has a high capacitance, and apotential at the charge storage node 44 stays nearly unchanged. For thisreason, a potential at the charge storage node 44′ of the second imagepickup cell 31′ for low noise is little affected by the charge storagenode 44.

Additionally, since the charge storage node 44 has a high capacitance,the potential at the charge storage node 44 is little affected by thecharge storage node 44′. For this reason, no distortion appears in animage even in the first image pickup cell 31 for a high level ofsaturation.

The second image pickup cell 31′ functions as an image pickup region forpickup of an image of a dark subject with a small amount of light. Adesirable characteristic required of the second image pickup cell 31′ issmall random noise. The number of saturation electrons may be small,that is, the second image pickup cell 31′ may have a low level ofsaturation.

As shown in FIGS. 7 and 8, the second pixel electrode 8 can achieve ahigh sensitivity with a relatively small area by using light condensingby the microlens 12. As shown in FIG. 3, a reduction in the area of thesecond pixel electrode 8 reduces the capacitance of the charge storagenode 44′ in the second image pickup cell 31′ and secures a relativelyhigh conversion gain for the amplification transistor 40′. In thefeedback circuit in FIG. 5, since the high conversion gain makes theoperation of the feedback circuit effective, random noise is effectivelysuppressed.

Additionally, in the feedback circuit in FIG. 5, since a hightransconductance gm of the amplification transistor 40′ increases thedrive capability of the transistor, random noise is suppressed moreeasily. In the present embodiment, the area of the second chargedetection circuit 51′ is set to be larger than that of the first chargedetection circuit 51. More specifically, a gate width of theamplification transistor 40′ in the second image pickup cell 31′ is setto be larger than that of the amplification transistor 40 in the firstimage pickup cell 31. This results in a high transconductance gm of theamplification transistor 40′.

Noise is relatively larger in the first image pickup cell 31 than in thesecond image pickup cell 31′. Note that an image obtained by the firstimage pickup cell 31 and an image obtained by the second image pickupcell 31′ are synthesized in high dynamic range processing. An improvedS/N ratio is obtained after the synthesizing, and noise resulting fromthe first image pickup cell 31 does not matter in a composite image.

FIG. 9 schematically shows areas occupied by the first charge detectioncircuit 51 and the second charge detection circuit 51′, respectively, inthe unit pixel 30. A region 60 represents the area of the first chargedetection circuit 51, and a region 61 represents the area of the secondcharge detection circuit 51′. The areas of the first and second chargedetection circuits 51 and 51′ each refer to the total sum of the areasof the transistors formed on the P-type silicon substrate 1. The areaoccupied by the transistors of the second charge detection circuit 51′can be increased by reducing the area occupied by the transistorsconstituting the first charge detection circuit 51. This results in ahigh transconductance gm of the amplification transistor 40′ in thesecond charge detection circuit 51′, which allows the second imagepickup cell 31′ to implement low-noise image pickup.

(Driving Method for Imaging Device 100)

An example of the operation sequence of the imaging device 100 will bedescribed with reference to FIG. 10.

FIG. 10 schematically shows exposure and readout operations during onecycle (one frame) period in the imaging device 100. The abscissarepresents time while the ordinate represents a readout row. FIG. 10shows how so-called rolling shutter readout is performed. In the imagingdevice 100, a dynamic range can be extended by performing exposure andreadout operation with the same timing using the first image pickup cell31 and the second image pickup cell 31′.

In the device configuration shown in FIG. 6, there is a sensitivitydifference of about one order of magnitude between the first imagepickup cell 31 and the second image pickup cell 31′. It is thus possibleto make the dynamic range about one order of magnitude higher than inthe case of common pixels even if exposure and readout is performed inthe same manner.

In the present embodiment, the first image pickup cell 31 and the secondimage pickup cell 31′ each have independent exposure and readout timingin order to further extend a dynamic range. In one cycle of image pickupoperation, the second image pickup cell 31′ is subjected to exposureduring a first storage time T1, and the first image pickup cell 31 issubjected to exposure during second storage times T2 and T3 shorter thanthe first storage time T1. A specific description will be given below.

In the present embodiment, one cycle has, for example, a length of 1/60seconds. In each second image pickup cell 31′, exposure is performedduring a storage time T1 close in length to one cycle. After a storagetime, charges in the second image pickup cells 31′ in each row aresequentially read out (readout 1). After completion of the readout foreach row, charges stored in all the second image pickup cells 31′ in therow to be read out are reset.

In the first image pickup cell 31, so-called non-destructive readout isperformed at least twice in one cycle. For example, first exposure isperformed during a storage time T2 which is 1/30 ( 1/1800 seconds) ofone cycle period, and readout is performed after completion of theexposure (readout 2). After that, second exposure is performed for astorage time T3 which is ½ ( 1/120 seconds) of one cycle period withoutresetting of stored charges, and readout is performed after completionof the exposure (readout 3). With the above-described operationsequence, three pieces of image pickup data different in exposure timecan be acquired in one cycle period. In a case where exposure andreadout is performed in the same manner, the dynamic range can be madehigher by about one order of magnitude. Synthesizing of the pieces ofimage pickup data allows the dynamic range to be made even higher byabout one and a half orders of magnitude. Thus, a high dynamic rangeimage with a dynamic range higher by about two and a half orders ofmagnitude in total can be generated.

Modifications of the imaging device 100 will be described below withreference to FIGS. 11A to 12B.

FIGS. 11A to 11C show variants, respectively, of the planar shape of thesecond pixel electrode 8. As shown in FIGS. 11A to 11C, the planar shapeof the second pixel electrode 8 need not be a circular shape. The planarshape of the second pixel electrode 8 may be, for example, a doughnutshape as shown in FIG. 11A, a cross shape as shown in FIG. 11B, or anotched shape as shown in FIG. 11C. Additionally, notches need not berectangular and may be circular. With such a shape, a change insensitivity of the first image pickup cell 31 due to a change in lightincident angle can be reduced. Even if the light incident angle changes,the ratio between the sensitivity of the second image pickup cell 31′and the sensitivity of the first image pickup cell 31 can be keptconstant.

Although an example in which incident light is condensed onto the centerof the unit pixel 30 using the microlens 12 has been illustrated in thepresent embodiment, the present disclosure is not limited to this. Theimaging device 100 need not include the microlens 12. If light is notcondensed, the sensitivity and the number of saturation electrons dependonly on the area of a pixel electrode and are approximately proportionalto the area of a pixel electrode. It is thus possible to omit themicrolens 12 and set a sensitivity ratio using only a pixel electrodearea ratio.

FIGS. 12A and 12B each show respective examples of the shapes of thepixel electrodes without the microlens 12. As shown in FIG. 12A, thesecond pixel electrode 8 may be arranged at the center of the unit pixel30, and the first pixel electrode 7 may be arranged around the secondpixel electrode 8 with a gap between the first pixel electrode 7 and thesecond pixel electrode 8. Alternatively, as shown in FIG. 12B, the firstpixel electrode 7 may be arranged at the center of the unit pixel 30,and the second pixel electrode 8 may be arranged around the first pixelelectrode 7 with a gap between the first pixel electrode 7 and thesecond pixel electrode 8. It suffices if the area of the second pixelelectrode 8 is larger than that of the first pixel electrode 7. Theshapes of the pixel electrodes can be arbitrarily determined.

Note that the above-described configurations reduce the area of thefirst pixel electrode 7 and lower both the cell sensitivity and thecapacitance. The capacitance of the charge storage node 44 in the firstimage pickup cell 31 can be increased by connecting the MOM capacitor 6to the first pixel electrode 7.

The present embodiment has illustrated an example in which the firstimage pickup cell 31 and the second image pickup cell 31′ are differentin random noise and the number of saturation electrons from each other.The present disclosure, however, is not limited to this, and the imagepickup cells may be different from each other in at least one of randomnoise and capacitance. In the present embodiment, the capacitance of thecharge storage node of the second image pickup cell 31′ is reduced, andthe conversion gain is set to be high, in order to suppress random noiseusing the feedback circuit. As a result, the number of saturationelectrons of the second image pickup cell 31′ can be made smaller. Notethat, to cancel out random noise by taking difference between pieces ofdata before and after image pickup using an external memory, thecapacitance of the charge storage node of the second image pickup cell31′ need not be reduced. Image synthesizing can be eased by connecting acapacitive element (for example, a MOM capacitor) to the second imagepickup cell 31′ to increase the number of saturation electrons. Forexample, if the areas of the first pixel electrode 7 and the secondpixel electrode 8 are equalized in a configuration without the microlens12, the first pixel electrode 7 and the second pixel electrode 8 areapproximately equal in sensitivity and capacitance. If a MOM capacitoris connected to the first pixel electrode 7 in this state, thecapacitance of the charge storage node 44 in the first image pickup cell31 increases. That is, the first image pickup cell 31 and the secondimage pickup cell 31′ can be made different only in capacitance. Thisdegrades sensitive performance but allows easy image synthesizing.

The term “storage capacitance” in the present disclosure refers to allcapacitive components connected to a pixel electrode. In the presentembodiment, a first storage capacitance is exemplified by the chargestorage node 44 including the MOM capacitor 6. A second storagecapacitance is exemplified by the charge storage node 44′. A capacitiveelement (capacitor) is exemplified by the MOM capacitor 6.

Second Embodiment

An imaging device 100 according to a second embodiment will be describedwith reference to FIGS. 13 to 16.

A unit pixel 30A according to the second embodiment is different fromthe unit pixel 30 according to the first embodiment in that a firstimage pickup cell 31 has a metal insulator metal (MIM) capacitor 13 as acapacitive element. The unit pixel 30A will be described below with afocus on differences from the unit pixel 30.

FIG. 13 schematically shows a cross-section of the device structure ofthe unit pixel 30A according to the present embodiment. The first imagepickup cell 31 has the MIM capacitor 13 as a capacitive element. The MIMcapacitor 13 is a laminated body including an upper electrode 14, alower electrode 16, and an insulator 15 located between the upperelectrode 14 and the lower electrode 16.

As the material for the insulator 15, a high dielectric material, suchas a silicon nitride film, hafnium oxide (HfO₂), zirconium oxide (ZrO₂),strontium titanate (SrTiO), or titanium oxide (TiO₂) is used. Note thata silicon nitride film is commonly used as a capacitance for an analogcircuit. Hafnium oxide (HfO₂) or zirconium oxide (ZrO₂) is used as thematerial for a capacitive insulating film of a dynamic random accessmemory (DRAM). If there is a leak current in the insulator 15, chargescaused by the leak current are stored in a charge storage node. As aresult, the leak current serves as a dark state noise component.

The composition of a film of a high dielectric material is likely tochange by thermal treatment after film formation. For example, thermaltreatment at about 400° C. may promote crystallization, and currentleakage characteristics may deteriorate. The MIM capacitor 13 is thusdesirably formed after completion of formation of a metal interconnect.Alternatively, the MIM capacitor 13 is desirably formed at as high aportion as possible of the metal interconnect. To form metalinterconnect including contact plugs using polysilicon or tungsten, afilm formation temperature of which exceeds 400° C., the formation isdesirably performed before formation of the MIM capacitor 13.

A material and a structure different from those for interconnect can beadopted for the MIM capacitor 13. The MIM capacitor 13 uses theinsulator 15 that is made of a material having a high dielectricconstant and has a thickness of several tens of nm and can secure acapacitance sufficiently larger than that of the MOM capacitor 6described in the first embodiment. Note that an additional process isrequired to form a MIM capacitive element. Note that manufacturing costsrise by an amount corresponding to the additional process. In contrast,if the MOM capacitor 6 is used as a capacitive element, an interconnectstructure used as interconnects for exchange of signals within a pixelor between pixels can be diverted. This allows suppression of a rise inmanufacturing costs. Note that, if an interconnect structure isdiverted, the capacitance density of the MOM capacitor 6 is limited bythe interconnect structure. If interconnects are densely arranged, aspace where the MOM capacitor 6 can be arranged is hard to secure, and asufficient capacitance may not be obtained. In such a case, the MIMcapacitor 13, an arrangement space for which can be secured irrespectiveof densely arranged interconnects, is desirably used as a capacitiveelement. The bottom line is that an optimum capacitive element may beappropriately selected in accordance with a design specification and thelike.

The MIM capacitor 13 increases the capacitance of a charge storage node44 of the first image pickup cell 31. As a result, the number ofsaturation electrons of the first image pickup cell 31 can be increased,like the first embodiment. The first image pickup cell 31 functions asan image pickup cell for a high level of saturation. Note that a secondimage pickup cell 31′ has the same structure as the second image pickupcell 31′ of the unit pixel 30 according to the first embodiment. Forthis reason, the second image pickup cell 31′ functions as an imagepickup cell for low noise.

FIG. 14 schematically shows the layout of a unit pixel 30B according toa modification of the present embodiment when viewed in plan view. Theunit pixels 30B for 3-by-3 pixels are shown in FIG. 14. FIG. 15schematically shows a cross-section of the device structure of the unitpixel 30B. FIG. 16 schematically shows a cross-section of the unit pixel30B taken along line XVI-XVI shown in FIG. 14.

The unit pixel 30B has the first image pickup cell 31 that has a firstpixel electrode 7 and the second image pickup cell 31′ that has a secondpixel electrode 8. Three unit pixels 30B are arranged along line XVI-XVIshown in FIG. 14 (a direction which is approximately 45° with respect toan x-axis in FIG. 14). The first pixel electrode 7 is locatedapproximately at the center of surrounding ones of the second pixelelectrodes 8 arranged in a grid pattern. The area of the second pixelelectrode 8 is larger than that of the first pixel electrode 7.According to the present modification, the first image pickup cells 31and the second image pickup cells 31′ can be densely arranged, and anincrease in layout efficiency is achieved.

As shown in FIG. 15, a microlens 12 is supported by a P-type siliconsubstrate 1 so as to cover a second photoelectric converter 43′, unlikethe first embodiment. The MIM capacitor 13 is arranged between a firstphotoelectric converter 43 and the second photoelectric converter 43′when viewed from a normal direction of the P-type silicon substrate 1.In other words, the MIM capacitor 13 is arranged between the first pixelelectrode 7 and the second pixel electrode 8. As shown in FIG. 15, theMIM capacitor 13 may be formed such that at least a part of the MIMcapacitor 13 overlaps with one or both of the first pixel electrode 7and the second pixel electrode 8. This increases the size of the MIMcapacitor 13, which allows an increase in the capacitance of the MIMcapacitor 13.

According to the present modification, the capacitance of the chargestorage node 44 of the first image pickup cell 31 can be increased, likethe second embodiment. This results in an increase in the number ofsaturation electrons of the first image pickup cell 31, which allows thefirst image pickup cell 31 to function as an image pickup cell for ahigh level of saturation.

FIG. 17 schematically shows a cross-section of the device structure of aunit pixel 30C according to another modification of the presentembodiment. Two microlenses 12 are supported by a P-type siliconsubstrate 1 so as to cover a first photoelectric converter 43 and asecond photoelectric converter 43′, respectively. The light condensingarea of the microlens of the second image pickup cell 31′ is larger thanthat of the microlens of the first image pickup cell 31. The arrangementof the microlens for the first image pickup cell 31 allows incidentangle characteristics of the first image pickup cell 31 and the secondimage pickup cell 31′ to be made uniform and allows acquisition of amore natural composite image.

The second image pickup cell 31′ includes a MIM capacitor 13′ which hasa smaller capacitance than that of the MIM capacitor 13. The purpose ofconnecting the MIM capacitor 13′ to a second charge storage node 44′ isas follows. A control voltage is applied to a terminal 55 on the sideopposite to a terminal connected to the second charge storage node 44′of the MIM capacitor 13′. The voltage of the second charge storage node44′ is controlled using a capacitive coupling via the MIM capacitor 13′,thereby suppressing random noise and a leak current.

For example, the capacitance of the second charge storage node 44′ whenthe MIM capacitor 13′ is not connected is 0.5 fF to 3 fF. Note that thecapacitance of a charge storage node depends largely on a pixel size. Ifthe MIM capacitor 13′ is connected, the capacitance of the MIM capacitor13′ is set to be enough to avoid an increase in random noise and not todrastically increase the capacitance of the second charge storage node44′. Note that the MIM capacitor 13 is used to increase the capacitanceof the first charge storage node 44. For this reason, the capacitance isset so as to exceed the capacitance of the first charge storage node 44when the MIM capacitor 13 is not connected. For example, the capacitanceof the first charge storage node 44 when the MIM capacitor 13 is notconnected is 0.5 fF to 3 fF.

Third Embodiment

An image pickup module 200 according to the present embodiment will bedescribed with reference to FIG. 18.

FIG. 18 schematically shows functional blocks of the image pickup module200 that is equipped with an imaging device 100.

The image pickup module 200 includes the imaging device 100 according tothe first embodiment and a digital signal processor (DSP) 300. The imagepickup module 200 processes a signal obtained by the imaging device 100and outputs the signal to the outside.

The DSP 300 functions as a signal processing circuit which processes anoutput signal from the imaging device 100. The DSP 300 receives adigital pixel signal output from the imaging device 100. The DSP 300performs, for example, gamma correction, color interpolation processing,space interpolation processing, auto white balance processing, and thelike. Note that the signal processing circuit may be a microcomputer orthe like which controls the imaging device 100 in accordance withvarious settings specified by a user and controls the overall operationof the image pickup module 200.

The DSP 300 processes a digital pixel signal output from the imagingdevice 100 and calculates optimum reset voltages (VRG, VRB, and VRR).The DSP 300 feeds back the reset voltages to the imaging device 100.Reference characters VRG, VRB, and VRR denote a reset voltage related toa G pixel, a reset voltage related to a B pixel, and a reset voltagerelated to an R pixel, respectively. Note that the reset voltages mayeach be a feedback signal which is transmitted from a feedback signalline 49 or a vertical signal line 45. The imaging device 100 and the DSP300 can also be manufactured as one semiconductor apparatus (so-calledsystem-on-a-chip (SoC)). This allows miniaturization of an electronicdevice using the imaging device 100.

Note that it is, of course, possible to commercialize only the imagingdevice 100 without modularization. In this case, a signal processingcircuit may be externally connected to the imaging device 100, andsignal processing may be performed outside the imaging device 100.Although the first and second embodiments have illustrated an example inwhich a photoelectric conversion film is arranged on the obverse side ofthe silicon substrate 1, and incident light from the obverse side issensed, the present disclosure is not limited to this. The presentdisclosure also includes a backside illumination (BSI) image sensor inwhich a photoelectric conversion film is arranged on the reverse side ofthe silicon substrate 1, and incident light from the reverse side issensed.

An imaging device according to the present disclosure is useful in animage sensor used in a camera, such as a digital camera or an in-carcamera.

The present disclosure further includes the imaging devices and drivingmethods below.

[Item 1]

A driving method for any one of the above-described imaging devices,including exposing a second image pickup cell in a second storage timeand exposing a first image pickup cell in a first storage time shorterthan the second storage time, during one cycle period.

The driving method according to Item 1 is capable of picking up an imageof a subject with a high dynamic range without blown out highlights andblocked up shadows.

[Item 2]

The driving method for the imaging device according to Item 1, in whichthe first image pickup cell is made to perform non-destructive readoutat least twice during the one cycle period.

The driving method according to Item 2 allows further extension of adynamic range.

[Item 3]

Any one of the above-described imaging devices, in which a firstcapacitive element has one pair of electrodes.

[Item 4]

Any one of the above-described imaging devices, in which a firstcapacitive element is a MOM capacitor or a MIM capacitor.

The imaging device according to Item 4 allows an increase in the numberof saturation electrons of the first image pickup cell.

[Item 5]

Any one of the above-described imaging devices, in which a plurality ofthe first image pickup cells and a plurality of the second image pickupcells are arranged in respective matrices, an area of the first pixelelectrode is smaller than an area of the second pixel electrode, andeach of the plurality of first image pickup cells is locatedapproximately at a center of a region defined by 2-by-2 image pickupcells among the plurality of second image pickup cells.

The imaging device according to Item 5 allows an increase in layoutefficiency.

What is claimed is:
 1. An imaging device comprising: a first imagepickup cell comprising a first photoelectric converter that convertsincident light into first charges, the first photoelectric converterincluding a first pixel electrode, a first electrode facing the firstpixel electrode, and a first photoelectric conversion region sandwichedbetween the first pixel electrode and the first electrode, and a firstcharge storage node that is coupled to the first pixel electrode, thefirst charge storage node accumulating the first charges; a second imagepickup cell adjacent to the first pixel, the second image pickup cellcomprising a second photoelectric converter that converts incident lightinto second charges, the second photoelectric converter including asecond pixel electrode, a second electrode facing the second pixelelectrode, and a second photoelectric conversion region sandwichedbetween the second pixel electrode and the second electrode, and asecond charge storage node that is coupled to the second pixelelectrode, the second charge storage node accumulating the secondcharges; and a first microlens located on an incident light side of thefirst photoelectric converter, the first pixel electrode being locatedon an optical axis of the first microlens, wherein the first pixelelectrode has a first area, and the second pixel electrode has a secondarea less than the first area.
 2. The imaging device according to claim1, wherein the first microlens does not cover the second pixelelectrode.
 3. The imaging device according to claim 2, wherein thesecond pixel electrode is not covered by a microlens.
 4. The imagingdevice according to claim 1, further comprising a second microlenslocated on an incident light side of the second photoelectric converter,the second microlens covering the second pixel electrode.
 5. The imagingdevice according to claim 4, wherein the first microlens has a firstcondensing area, and the second microlens has a second condensing arealess than the first condensing area.
 6. The imaging device according toclaim 1, wherein the first microlens covers the second pixel electrode.